Handle absent lzma library

[stephenfin]

The lzma library may be missing in some environments as it is an optional dependency [1]. Fall back to bz2 in this instance.

[1] https://devguide.python.org/getting-started/setup-building/

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 97.72%. Comparing base (afa8019) to head (ff46021).
Report is 13 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd2/cmd2.py	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1318      +/-   ##
==========================================
- Coverage   98.52%   97.72%   -0.81%     
==========================================
  Files          22       22              
  Lines        5782     5753      -29     
==========================================
- Hits         5697     5622      -75     
- Misses         85      131      +46     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[kmvanbrunt]

@osfrickler I went ahead and made the suggested changes since the function needed to be refactored a bit.
Let me know if my changes match your intentions.